Mechanical Polymer Testing

We provide excellent testing services for mechanical polymers in a short time at a competitive price. Now you might think this is a sublime aspiration. But how can we fulfill this promise? On the one hand, we focus on static testing of materials using a very well-equipped and fully programmable electromechanical test frame. On the other hand, we furthered methods of video extensometry, thus assuring a comprehensive, precise and reliable strain measurement. In addition, we have implemented a specimen and data tracking system, that improves efficiency and eliminates human errors. Finally, we developed a system for comprehensive and efficient computerized reporting. So, you get a high-quality test report suitable for further automatic data processing. This makes us a specialized testing partner for simulation engineers, product developers and material producers.



Tests That we Provide

On one hand, we test rigid, reinforced polymers including short fiber reinforced polymers (SFRP), long fiber reinforced polymers (LFRP) and sheet molding compounds (SMC). On the other hand, we test soft polymers including elastomers and thermoplastics. For all these materials we provide appropriate tensile tests, compression tests, bend tests, creep tests, creep-to-rupture tests, relaxation tests, relaxation-retraction tests and cyclic characterization tests. We can perform all tests at temperatures between -80°C and 250°C. And we measure strain and transverse strain with precision.

Instrumentation

We measure and control force from 0.01 N to 10 kN. On the one hand, this allows us to test very small and soft samples. On the other hand, this is enough for testing rigid plastics such as SFRP, SFRP and SMC materials. Our current upper limit of 10 kN is due to our polymers-oriented load frame.

We measure and control strain using a programmable video extensometer. It is fully integrated into the control and data collection of our load frame. Consequently, we can measure strains from 0.001% to the very large values observed in thermoplastics and elastomers. Also, we can freely choose gauge lengths and locations. Therefore, we carry out all our tests over the entire temperature range, with optimized measurement of strain and transverse strain. Not to forget, we are able to do tests in strain control whenever necessary. The maximum strain rate with standard specimens is approximately 10%/s.

We measure and control temperature using our temperature chamber. It is fully integrated into the control and data acquisition of our load frame. We use these thermomechanical capabilities for our research work. During ordinary testing of polymers, the ability to see deformation and temperature changes in real time helps us to understand the physics of the polymer and, thus, improves the quality of the tests.

Specimen Preparation

Send us your specimens

The photo shows a cnc mill inside an enclosure and a computer monitor to the left. Both stand on a table. Below the table are a computer, a controller and a suction unit.
CNC mill

If you send “dog bones” with a width of less than 40 mm and a thickness of up to 8 mm, they will fit our preferred pneumatic grips. If you send larger dog bone type specimens, we will use our mechanical screw grips. When it comes to bending samples, we usually work with geometries proposed by the ASTM and ISO standards. When it comes to compression specimens from soft polymers, we take care to manufacture them with high precision. For compressive testing of rigid, reinforced specimens, we use the ASTM D695 support jig. Therefore, we prefer ASTM D695 specimens.

Send us plates or parts

Send us plates or parts, then we will make specimens for you. On the one hand, if you send a soft sheet material and you need standard ASTM or ISO samples, we will use a die-cutter. In this case, the production of samples will be economical, and the cut will be flawless. On the other hand, if you send components or rigid plates, we will process the samples using our CNC mill. We have a database of sample geometries and an efficient and quality assured CAD / CAM process.

Let us design specimens

The figure shows a 3D CAD model of a specimen with short shoulders and short tabs. There are 10 strain marks on its parallel length. They are arranged in two times 5 points, spread over the parallel length on both sides.
3D CAD model of a custom specimen with strain marks

Let us design specimens, because standard specimens do not always fit the purpose. When we cut specimens from components, size restrictions may require non-standard geometries. On the other hand, sometimes standard specimens simply do not suit the material. For example, brittle materials may be sensitive to stress concentrations, requiring optimized shoulders and tabs. We develop specimens in an iterative two-step process. First, we develop a new specimen using parametric CAD and simulate its mechanical behavior during testing. After that, having a promising virtual candidate, we make a prototype. Using our proven CAM-CNC production route, we are fast at manufacturing prototypes and testing them. Therefore, the creation of new specimen geometries is only a matter of hours and days.

Conditioning, Humidity and Aging

Temperature and time

Preconditioning most materials at room temperature for limited time will influence mechanical properties little. However, preconditioning them at elevated temperatures for the same time can change them significantly by physical aging. These effects are important for two reasons. First off, if preconditioning is not uniform for all tests during elevated temperature testing, this causes inconsistency and scatter of results. Second, if a component experiences physical aging during lifetime, this can alter its behavior and should be regarded during testing. Therefore, we plan, monitor and record preconditioning carefully. For elevated temperatures we use a temperature chamber. And we monitor and record the laboratory temperature at 23 ° C.

Humidity of Polyamides

We usually condition polyamides in three states. First off, we come to equilibrium in standard conditions at 23°C and a relative humidity of 50%. For this, we use accelerated conditioning using salt solutions and elevated temperatures. This standard condition is most useful for testing at lower temperatures, where specimen humidity is not influenced by heat-up and soaking during the mechanical test procedure. Secondly, we condition to the dry state. In doing so, we store specimens in a heat chamber, monitor the water content in the air and set the temperature to achieve the desired low residual humidity. This condition is suitable for immediate testing at room temperature. And this is good for testing at elevated temperatures near the conditioning temperature, where the water content in the sample is already in equilibrium with the atmosphere. Thirdly, we bring the polyamide to a wet condition using boiling water. Then we test the specimens at standard conditions (23°C, 50%rH). We monitor and record laboratory moisture using a calibrated hygrometer. And we monitor and measure the moisture of a material by measuring the weight of samples during conditioning and storage. In addition, we store the material under controlled conditions using desiccators and humidity-tight packaging. Finally, we make sure that the humidity is well defined, homogeneous and repeatable during plastics testing. We do this by diffusion simulation using finite element analysis.

Aging

We are equipped to age the material at elevated temperatures in air, in solutions and in oil. Depending on the medium, we can go to a maximum temperature of 250°C. As a rule, the aging times reach over some orders of magnitude, whereby a reasonable maximum time is 1000 hours.

Testing Process and Quality Assurance

Computerized flow of data and material

The grayscale picture shows a polymer specimen. On its parallel length, there are four strain marks. On the left and right side, the specimen is tagged with a sticker, containing datamatrix code, name and tortuetec logo. The picture has a scale bar, indicating 10 mm.
Specimen with datamatrix labels

Firstly, when you send us parts and specimens, we name them, mark them with machine readable data matrix codes, assign them to a fixed place in the storage, and register them in a database. Then, each process with your specimens and parts is the sequence of getting them from the storage, scanning their data matrix code, performing a mostly computerized process and returning them to storage.

At this, typical processes are heat treatment, CNC machining, dimensional measurements and mechanical testing. For all processes, the software takes care of reading the data matrix code, opening the connection to the database, getting the process parameters from the database, managing and controlling the process, and writing the results and measurements back to the database. In general, we maximize repeatability and transparency and minimize the chance of human error.

Verification and Validation built into the process

On the right, the image shows a red-yellow-green-blue scale bar for von Mises stress. Also, there are six contour plots of von Mises stress on specimens.
FE computation of stress concentrations

We have a real-virtual laboratory that closely combines physical experiments and computer simulations. Thereby, we have included verification and validation in our standard testing process. Here we want to show it with a few examples.

  • When we receive specimens, we generate a 3D model, including the planned specimen geometry and the strain measurement setup. Then we measure the actual specimen dimensions. So, if there is a deviation between the planned and actual specimens, we will know it at an early stage.
  • Then we use the 3D model for finite element simulation of the test. For example, we assess stress concentration factors or compression stability. So, if a specimen appears not to be optimal or even inadequate for the intended test, we give immediate feedback
  • During mechanical testing, we measure the strain in at least two places of the specimen. For example, we measure the longitudinal deformation on the left and right side. Thus, we get an idea of the uniformity of strain in real time. Accordingly, we will respond immediately.

Summarizing, we can state that our verification and validation steps improve the quality of testing, reduce the time it takes to complete a project, and greatly simplify the correct interpretation of test results.

Calibration of measurement devices

Our mechanical and thermal measurement devices and machines are calibrated regularly. Your calibration partner is DAKKs accredited.

Test Report

Reports for people and machines

Our test reports are generated from the previously mentioned database using a modular computer program. There is no direct human intervention at the time of report compilation. Therefore, the reports are fully repeatable, quality guaranteed and suitable for further use in automatic mode. Besides, the report is provided as a modular Excel document that people and computer algorithms alike can easily access. For each report module, the first Excel sheet contains descriptions, the second sheet contains data. Thereby, using our report, you can quickly find comprehensive test data and its context.

This picture shows a typical specimen description, which we use in our test reports. We include a specimen drawing with dimensions of specimen geometry and extensometry setup. Also, we give explanations about measurements.
Example of a specimen description given in the test report

Report modules structured by usage

To start with, our statistical evaluation module is the best way to get the aggregated big picture. For example, if tensile tests with 10 repeats were performed at some temperature levels, this module shows aggregated results per temperature in tables and figures at a glance. Then, if you are interested in the details, you find report modules about each tested part and specimen, about testing parameters and test results. Further, if you are a simulation engineer, you may be interested in more detailed data. Therefore, for each test you get a sheet with a table of time dependent data of (e.g. stress, strain, …) and charts. Additionally, indices help you with filtering and extracting data. And not to forget, we add a photo showing the specimen after testing, so you can see deformation and damage.

Transparent specimen documentation using Storage Boards

During the testing project, we store the specimens on computer-controlled, fixed storage positions. These storage positions are located on white polymer sheets of size A3. We call them Storage Boards. At the end of the testing campaign, we weld a transparent cover to each storage board, covering and fixing each test specimen. Then we place all storage boards in labeled archive boxes. This allows you to find tested specimens quickly and easily as well on a Storage Board as in the Excel report. So, if you want to further explore or show your tested specimens, you can do it beautifully and without problems.

The picture shows a white board on a gray background. On the board, marks indicate storage locations for specimens and most positions are occupied. On the bottom, name and a datamatrix code identify the sorage board
Computer administered storage ofspecimens using Storage Boards